Lithium-manganese complex oxides, including lithium manganate composed mainly of manganese and having a spinel structure, are being studied as positive electrode active materials for lithium ion secondary batteries (hereunder referred to as “LIB”) (see Non-Patent Literature 1, for example).
Lithium-manganese complex oxides not only have high rate capability and high safety, but they are also inexpensive. Therefore, lithium-manganese complex oxides are being studied for application not only in portable electronic devices but also for vehicles such as hybrid vehicles (HV) and electric vehicles (EV). However, LIB employing a lithium-manganese complex oxide as the positive electrode active material has low discharge capacity per volume, it's so called energy density, than LIB that has lithium cobaltate as the positive electrode active material. It is therefore desired to improve the energy density of lithium-manganese complex oxides.
Improving the energy density of a lithium-manganese complex oxide requires increase in its packing property. The packing property of a lithium-manganese complex oxide is significantly affected by the packing property of the manganese compound starting material. In order to obtain a lithium-manganese complex oxide with a high packing property, a manganese compound with a high packing property may be used as the manganese starting material.
Manganese compounds with high packing properties include electrolytic manganese dioxide and its heat-treated products, and these are therefore most widely used as manganese starting materials for lithium-manganese complex oxides (see Patent Literature 1, for example).
In order to further improve the packing property of electrolytic manganese dioxide, it has been proposed to use γ-type electrolytic manganese dioxide having a BET specific surface area of no greater than 35 m2/g, electrolytically synthesized in a manganese sulfate solution containing suspended manganese oxide, as the manganese starting material (Patent Literature 2).
For industrial production of such electrolytic manganese dioxide, electrolysis is performed using an electrolyte solution obtained from a starting material such as manganese ore or industrial water. To produce electrolytic manganese dioxide that has excellent reactivity with lithium compounds and the like, it is necessary to inhibit incorporation of impurities derived from these starting materials into the electrolytic manganese dioxide. Industrial methods for preventing inclusion of starting material-derived impurities have been reported, including a method wherein the starting material is a manganese compound obtained by extraction removal of the impurities from manganese ore (Patent Literature 3), and a method of using an electrolyte solution wherein the magnesium has been removed by adding a fluoride during the electrolyte solution treatment step (Patent Literature 4).